| 1. |
- Beccu, E., et al.
(författare)
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Efficiency of percussive drilling of rock with dissipative joints
- 1990
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Ingår i: International Journal of Impact Engineering. - 0734-743X .- 1879-3509. ; 9:3, s. 277-287
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Tidskriftsartikel (refereegranskat)abstract
- The nonlinear dissipative spring mass (NDSM) model for a percussive drill rod joint of the coupling sleeve (CS) type has been implemented into a Modula-2 program with the aid of which percussive drilling of rock is simulated. Transmission and dissipation of energy are first studied when a rectangular stress wave, generated through the impact by a uniform hammer, is transmitted through a single joint. The efficiency of energy transmission increases from 81 to 94% and the relative energy dissipation decreases from 8 to 1 or 2% when the length of the hammer varies from relatively short to relatively long. The effect of the joint preload is weak in the range from medium to relatively high preload. The efficiency of the percussive drilling process decreases with the number of joints but depends little on the joint preload. For soft rock, the efficiency increases with hammer length, whereas for medium and hard rock the dependence of efficiency on hammer length is not monotonic. This is because soft rock requires a long incident wave for efficient conversion of energy to work at the bit, whereas the reverse is true for hard rock. It is also found that the efficiency of the percussive drilling process may be considerably underestimated if the effects of each joint on the length and shape of the transmitted wave and of multiple reflections within the drill string are neglected
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| 2. |
- Beccu, R., et al.
(författare)
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Transmission and dissipation of stress wave energy at a percussive drill rod joint
- 1987
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Ingår i: International Journal of Impact Engineering. - 0734-743X .- 1879-3509. ; 6:3, s. 157-173
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Tidskriftsartikel (refereegranskat)abstract
- Transmission and dissipation of stress wave energy at a percussive drill rod joint is determined for three joint types, three joint preload levels, three hammer lengths ( incident wave lengths) and a range of impact velocities ( incident wave amplitudes). The energies of the incident, reflected and transmitted waves are evaluated from measured strains, and then the dissipated energy is determined as the deficit of energy in the two latter waves compared with the first. The accuracy needed is obtained by introducing a compensation factor and determining its value from the requirement of momentum conservation. The experimental results for threaded standard joints are compared with experimental results for threadless dummy joints, made in one piece, and also with theoretical results. The latter are based on one model which represents the joint as a well on a one- dimensional elastic rod (characteristics impedance or CI model) and on another model which represents the joint as a rigid mass between one-dimensional elastic rods (rigid mass or RM model). For the joints 70-100% of the incident wave energy is transmitted and 0-20% is dissipated. The corresponding figures for the dummies are 90-100% and (±)1%, repectively. The latter figure indicates the inaccuracy in the measurement and evaluation procedures. The two models agree fairly well with the dummies. Under certain conditions they also give reasonably accurate predictions for the energy transmitted through a joint. As they sometimes give quite inaccurate results for energy transmission and furthermore fail to predict energy dissipation they need be refined.
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| 3. |
- Juntikka, Rickard, et al.
(författare)
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Weight-balanced drop test method for characterization of dynamic properties of cellular materials
- 2004
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Ingår i: International Journal of Impact Engineering. - 0734-743X .- 1879-3509. ; 30:5, s. 541-554
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Tidskriftsartikel (refereegranskat)abstract
- A novel weight-balanced drop rig used to evaluate the response of cellular materials subject to dynamic compression is presented. The testing method utilizes approximately constant velocity throughout the major part of the compression phase and the results compare well with results from other methods, reported in the literature. The repetitiveness is excellent, the rig is simple and the results are easily extracted. The applicability of the method for determination of elastic modulus is however limited to materials with relatively low stiffness. Accurate modulus measurements for stiff materials at high strain-rates require a very rigid and lightweight test set-up.
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| 4. |
- Lundberg, Bengt, et al.
(författare)
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Nonlinear dissipative spring mass model for a percussive drill rod joint of the coupling sleeve type
- 1989
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Ingår i: International Journal of Impact Engineering. - 0734-743X .- 1879-3509. ; 8:4, s. 303-313
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Tidskriftsartikel (refereegranskat)abstract
- A nonlinear dissipative spring mass (NDSM) model for a percussive drill rod joint of the coupling sleeve (CS) type is established. Such a joint consists of a cylindrical coupling sleeve with internal thread which connects two drill rods with external threads at their ends. The model disregards wave motion in the coupling sleeve but accounts for axial mobility of the sleeve relative to the rods. This mobility is due to local deformation and slip of the threads. The model is characterized by the mass of the sleeve and by three parameters which represent the coupling between the sleeve and the drill rods through the threads. A static and a dynamic test have been developed for determining the coupling parameters. The model is validated by simulating previous impact tests with a commercial CS joint.
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| 5. |
- Lundberg, Patrik, et al.
(författare)
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Impact of metallic projectiles on ceramic targets : transition between interface defeat and penetration
- 2000
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Ingår i: International Journal of Impact Engineering. - 0734-743X .- 1879-3509. ; 24:3, s. 259-275
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Tidskriftsartikel (refereegranskat)abstract
- Armour systems capable of defeating an incoming projectile on the surface of a ceramic have been reported by several authors. This capability, called interface defeat, signifies that the projectile material is forced to flow radially outwards on the surface of the ceramic without penetrating significantly. In order to investigate the conditions for interface defeat, two models for the interaction of a metallic projectile and a ceramic target were established. With the aid of them, upper and lower bounds for the transition impact velocity between interface defeat and normal penetration were estimated for a given combination of metallic projectile and ceramic target. These approximate bounds were found to be consistent with transition velocities determined experimentally for two projectile materials (tungsten and molybdenum) and five target materials (two types of silicon carbide, boron carbide, titanium diboride and a polycrystalline diamond composite).
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| 6. |
- Lundberg, Patrik, et al.
(författare)
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Influence of scale on the penetration of tungsten rods into steel-backed alumina targets
- 1996
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Ingår i: International Journal of Impact Engineering. - 0734-743X .- 1879-3509. ; 18:4, s. 403-416
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Tidskriftsartikel (refereegranskat)abstract
- As ballistic tests are often performed in reduced geometrical scale, the scaling laws are important for the interpretation of the results. In this study, we tested the validity of replica scaling, by which we mean that all geometrical dimensions are scaled uniformly, while the materials and the impact velocity are kept the same. Long tungsten projectiles with length-to-diameter ratio 15 were fired against unconfined alumina targets with steel backing. The tests were carried out with impact velocities 1500 m s−1 and 2500 m s−1, and in three different scales with projectile lengths 30, 75 and 150 mm (diameters 2, 5 and 10 mm). The alumina targets were photographed by means of a high-speed camera, and the tungsten projectiles were photographed inside the alumina targets by means of flash radiography. Also, the residual penetrations in the steel backings were measured. The Johnson-Holmquist model for ceramic materials was implemented into the AUTODYN code, which was used for simulation of the experiments. The agreement between results of experiment and simulation was fair, and over the tested interval of scales replica scaling was found to be valid with reasonable accuracy.
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| 7. |
- Nordlund, Erling, et al.
(författare)
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Slip and energy dissipation in conical friction joints loaded by impact-induced waves
- 1986
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Ingår i: International Journal of Impact Engineering. - 0734-743X .- 1879-3509. ; 4:2, s. 127-144
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Tidskriftsartikel (refereegranskat)abstract
- Conical friction (CF) joints subjected to impact-induced wave (IIW) loading are studied theoretically and experimentally. The system considered consists of a long, straight, cylindrical and linearly elastic rod which is impacted axially at one end, and which has a body attached by means of a CF joint at the opposite end. The conditions are such that the wave propagation is 1-D in the rod and can be neglected in the attached body, which is therefore treated as rigid. Slip and energy dissipation in the CF joint due to a single IIW load are determined as functions of four dimensionless parameters which represent the duration of the incident wave, the mass of the attached body, the behaviour of the CF joint and the initial slip due to a compressive preload. The conditions under which detachment occurs are also found. Results for repeated noninteracting IIW loads are derived from those obtained for a single IIW load. The theoretical results exhibit phenomena which may sometimes have a spectacular appearance. For example, four consecutive and identical IIW loads may have the effect of fastening, loosening, fastening again and, finally, detaching the CF joint. There is a fair agreement between theoretical and experimental results.
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| 8. |
- Nordlund, Erling
(författare)
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Slip and energy dissipation in cylindrical friction joints loaded by impact-induced waves
- 1987
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Ingår i: International Journal of Impact Engineering. - 0734-743X .- 1879-3509. ; 6:1, s. 35-50
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Tidskriftsartikel (refereegranskat)abstract
- Cylindrical friction (CYLF) joints subjected to impact-induced wave (IIW) loading are studied theoretically and experimentally. The system considered consists of a long, straight, cylindrical and linearly elastic rod which is impacted axially at one end by a cylindrical and linearly elastic hammer. A body is attached at the other end by means of a CYLF joint. The conditions are such that the wave propagation is 1-D in the hammer and the rod and can be neglected in the attached body which is therefore treated as rigid. The agreement between numerical and analytical solutions is good. There is also fair agreement between theoretical and experimental results.
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| 9. |
- Westerling, Lars, et al.
(författare)
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Tungsten long-rod penetration into confined cylinders of boron carbide at and above ordnance velocities
- 2001
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Ingår i: International Journal of Impact Engineering. - 0734-743X .- 1879-3509. ; 25:7, s. 703-714
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Tidskriftsartikel (refereegranskat)abstract
- The purpose was to investigate the influence of impact velocity and confinement on the resistance of boron carbide targets to the penetration of tungsten long-rod projectiles. Experimental tests with impact velocities from 1400 to 2600 m/s were performed using a two-stage light-gas gun and a reverse impact technique. The targets consisted of boron carbide cylinders confined by steel tubes of various thicknesses. Simulations were carried out using the AUTODYN-2D code and Johnson–Holmquist's constitutive model with and without damage evolution. The experimental results show that the penetration process had different character in three different regions. At low-impact velocities, no significant penetration occurred. At high-impact velocities, the relation between penetration velocity and impact velocity was approximately linear, and the penetration was steady and symmetrical. In between, there was a narrow transition region of impact velocities with intermittent and strongly variable penetration velocity. In the lower part of this region, extended lateral flow of the projectile took place on the surface of the target. The influence of confinement on penetration velocity was found to be small, especially at high-impact velocities. The simulated results for penetration velocity versus impact velocity agreed fairly well with the experimental results provided damage evolution was suspended below the transition region.
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| 10. |
- Ödéen, Sven, et al.
(författare)
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Prediction of impact force by impulse response method
- 1991
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Ingår i: International Journal of Impact Engineering. - 0734-743X .- 1879-3509. ; 11:2, s. 149-158
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Tidskriftsartikel (refereegranskat)abstract
- A method has been established which permits prediction of impact force history from the velocity response of each impacting body to an impulsive force applied to its impact face, and the impact velocity. The bodies may consist of one or several linearly elastic or viscoelastic materials. However, the method is limited to cases of impact without significant effects of friction and slip, with constant contact area and with small deformations. It has been applied to four cases of axial impact of a truncated cone or a compound cylinder on a long cylindrical rod. The truncated cone was made of Nylon-6, the compound cylinder of Nylon-6 and aluminum, and the long cylindrical rod of steel. For the truncated cone and the compound cylinder, measured as well as theoretically predicted impulse responses were used, while for the long cylindrical rod only a theoretically predicted impulse response was employed. In all cases good agreement was obtained between the impact force histories predicted, using the impulse response method, and those measured with the aid of strain gauges on the long cylindrical rod. Because of three-dimensional effects the best agreement was obtained for the predictions based on measured impulse responses for the truncated cone and the compound cylinder
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| 11. |
- Aare, Magnus, et al.
(författare)
-
Evaluation of head response to ballistic helmet impacts using the finite element method
- 2007
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 34:3, s. 596-608
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Tidskriftsartikel (refereegranskat)abstract
- Injuries to the head caused by ballistic impacts are not well understood. Ballistic helmets provide good protection, but still, injuries to both the skull and brain occur. Today there is a lack of relevant test procedure to evaluate the efficiency of a ballistic helmet. The purpose of this project was (1) to study how different helmet shell stiffness affects the load levels in the human head during an impact, and (2) to study how different impact angles affects the load levels in the human head. A detailed finite element (FE) model of the human head, in combination with an FE model of a ballistic helmet (the US Personal Armour System Ground Troops' (PASGT) geometry) was used. The head model has previously been validated against several impact tests on cadavers. The helmet model was validated against data from shooting tests. Focus was aimed on getting a realistic response of the coupling between the helmet and the head and not on modeling the helmet in detail. The studied data from the FE simulations were stress in the cranial bone, strain in the brain tissue, pressure in the brain, change in rotational velocity and translational and rotational acceleration. A parametric study was performed to see the influence of a variation in helmet shell stiffness on the outputs from the model. The effect of different impact angles was also studied. Dynamic helmet shell deflections larger than the initial distance between the shell and the skull should be avoided in order to protect the head from the most injurious threat levels. It is more likely that a fracture of the skull bone occurs if the inside of the helmet shell strikes the skull. Oblique ballistic impacts may in some cases cause higher strains in the brain tissue than pure radial ones.
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| 12. |
- Balieu, Romain, et al.
(författare)
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Damage at high strain rates in semi-crystalline polymers
- 2015
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 76, s. 1-8
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Tidskriftsartikel (refereegranskat)abstract
- A specific damage characterization method using Digital Image Correlation for semi-crystalline polymers is proposed for a wide range of strain rates. This damage measurement is an extension of the SEE method [16] which was developed to characterize the behaviour laws at constant strain rates of polymeric materials. This procedure is compared to the well-known damage characterization by loss of stiffness technique under quasi-static loading. In addition, an in-situ tensile test, carried out in a microtomograph, is used to observe the cavitation phenomenon in real time. The different ways used to evaluate the damage evolution are compared and the proposed technique is also suitable for measuring the ductile damage observed in semi-crystalline polymers under dynamic loading.
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| 13. |
- Burgert, J., et al.
(författare)
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Recursive algorithm for the distribution of characteristic impedance of an elastic rod producing a prescribed impact force history on a given elastic rod
- 2022
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 161
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Tidskriftsartikel (refereegranskat)abstract
- Axial impact between two elastic rods with piecewise constant characteristic impedance is considered. The distribution of characteristic impedance of the impacted rod and the impact velocity are given. By use of a 1D recursive algorithm, the characteristic impedances of the impacting rod are determined as long as they are all positive so that a prescribed impact force is realized. A condition for positivity of the characteristic impedance is derived in terms of transmission coefficients for wave energy. At the time from which positivity of all characteristic impedances cannot be maintained, or earlier, the characteristic impedance of the impacting body is continued, e.g., at the level of the last positive characteristic impedance or at level zero corresponding to cutting off the impacting rod. Examples of prescribed exponentially decreasing and linearly increasing impact forces are presented for a uniform impacted rod with constant characteristic impedance. In these examples, there is good agreement between the prescribed impact force and the impact force obtained from 3D FE simulation with a piecewise linear diameter of the impacting body that approximates the piecewise constant diameter obtained by use of the 1D recursive algorithm.
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| 14. |
- Bussac, Marie-Noёl, et al.
(författare)
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Viscoelastic impact between a cylindrical striker and a long cylindrical bar
- 2008
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 35:4, s. 226-239
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Tidskriftsartikel (refereegranskat)abstract
- Axial impact between a cylindrical striker of finite length and a long cylindrical bar, both of linearly viscoelastic materials, is considered. General results are derived for the impact force, the particle velocity and the strain in the bar in terms of closed-contour integrals. Such results are derived also for the transfer of momentum and energy from the striker to the bar. Numerical results for a striker and a bar made of the same material but with different cross-sectional areas are compared. In viscoelastic impact, unlike elastic impact, the duration of contact may be finite and larger than two transit times for a wave front through the striker due to the formation of a tail of finite length after the main pulse. Furthermore, multiple contacts and separations of the striker and the bar may occur within a range of striker-to-bar characteristic impedance ratios. In the case of viscoelastic impact studied numerically, the duration of contact is at least as long and the momentum and energy transferred are at most as large as in elastic impact. Strains measured at three locations of a polymethyl methacrylate (PMMA) bar impacted by strikers of the same material as the bar agree well with the theoretical results.
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| 15. |
- Börvik, Tore, et al.
(författare)
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Experimental and numerical study on the perforation of AA6005-T6 panels
- 2005
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 32:1-4, s. 35-64
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Tidskriftsartikel (refereegranskat)abstract
- Extruded aluminium panels find their use in many different structures such as offshore platforms, bridge decks, train and ship components and lightweight protection systems. Impacts or other types of high-speed loading conditions are thus a relevant issue for several of these applications. There are, however, not many investigations published on the perforation of extruded aluminium panels covering experiments in combination with numerical analyses. This paper presents an experimental and numerical study on the perforation of AA6005-T6 aluminium panels impacted by ogival-nosed steel projectiles. The chosen panel has three triangular-shaped cells with a total depth of 130 mm. The wall thickness is 6 mm in the front and rear side plate, and 3 mm in the slanting webs. A rather comprehensive material test programme has been carried out in order to determine the material's response to dynamic loading. The experimental results were used to calibrate slightly modified versions of the Johnson-Cook constitutive relation and fracture criterion. 3D numerical simulations of the perforation process were then performed on a high-performance computer using the MPP version of the explicit finite element code LS-DYNA. In the simulations, the typical dimension of the elements was less than 1 mm. The numerical model is able to capture the main trends in the experiments in an adequate manner, and excellent agreement between numerical and experimental results is obtained.
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| 16. |
- Forsberg, Jimmy, et al.
(författare)
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Evaluation of response surface methodologies used in crashworthiness optimization
- 2006
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 32:5, s. 759-777
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Tidskriftsartikel (refereegranskat)abstract
- Optimization of car structures is of great interest to the automotive industry. This work is concerned with structural optimization of a car body with the intent to increase the crashworthiness properties of the vehicle or decrease weight with the crashworthiness properties unaffected. In this work two different methodologies of constructing an intermediate approximation to the optimization problem are investigated, i.e. classical response surface methodology and Kriging. The major difference between the two methodologies is how the residuals between the true function value and the polynomial surface approximation value at a design point are treated.Several different optimization problems have been investigated, both analytical problems as well as finite element impact problems.The major conclusion is that even if the same kind of updating scheme is used both for Kriging and linear classic response surface methodology, Kriging improves the sequential behaviour of the optimization algorithm in the beginning of the optimization process. Problems may occur if a constraint is violated after several iterations and then classic response surface methodology seems to more easily be able to find a design point which satisfies the constraint.
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| 17. |
- Godio, Michele, et al.
(författare)
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Low-velocity out-of-plane impact tests on double-wythe unreinforced brick masonry walls instrumented with optical measurements
- 2023
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Ingår i: International Journal of Impact Engineering. - 0734-743X .- 1879-3509. ; 178
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Tidskriftsartikel (refereegranskat)abstract
- Unreinforced brick masonry makes up today a significant piece of the European built environment, including not only residential buildings but also strategically important structures that are not designed to withstand blasts and impacts. Yet, it is difficult to accurately estimate the response of these structures and the extent of damage they sustain during such extreme loading conditions. This paper presents the implementation and discusses the results of laboratory impact tests conducted on natural-scale double-wythe unreinforced brick masonry walls, a typology that is frequently found in Northern Europe. The walls were spanning vertically between two reinforced concrete slabs and were subjected to low-velocity drop-weight pendulum tests in which they were repeatedly hit until the opening of a breach in the center of the wall. The tests were instrumented with both hard-wired and optical measurements, the latter consisting of high-speed cameras and digital image correlation techniques, to face the difficulty of observing cracks and determining the deflections of the walls with adequate accuracy at the time of the impact. Investigated in these tests were the out-of-plane response of the walls and their capacity to resist the impacts. The axial load applied on the top of the walls was varied for two wall configurations and monitored throughout the tests to study the effect of arching on the failure mechanism produced and number of repeated hits needed to open the breach. Of interest was also the evidence of cracking, more specifically the way it initiated on the undamaged walls and next propagated upon consecutive hits. The data generated from these tests are made available to support further investigations on unreinforced masonry structures subjected to extreme actions.
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| 18. |
- Gustafsson, Gustaf, et al.
(författare)
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Fracture probability modelling of impact-loaded iron ore pellets
- 2017
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Ingår i: International Journal of Impact Engineering. - : Elsevier. - 0734-743X .- 1879-3509. ; 102, s. 180-186
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Tidskriftsartikel (refereegranskat)abstract
- Blast furnace iron ore pellets are sintered, centimetre-sized ore spheres with a high iron content. Together with carbonized coal, iron ore pellets are used in the production of steel. In transporting pellets from pelletizing plants to customers, iron ore pellets are exposed to different static and dynamic loading situations, resulting in strength degradation and, in some cases, fragmentation. This can lead to a reduced gas flow in the blast furnace, which causes reduced quality in steel production. Reliable numerical simulations that can predict the ability of the pellets to endure their handling are important tools for optimizing the design of equipment for iron ore handling. This paper describes the experimental and numerical work performed to investigate the impact fracture behaviour of iron ore pellets at different strain rates. A number of split Hopkinson pressure bar tests with different striker velocities are carried out and analysed to investigate the strain rate dependency of the fracture strength of iron ore pellets. Fracture data for iron ore pellets are derived and expressed in terms of statistical means and standard deviations. A stress based, strain-rate dependent fracture model that takes triaxiality into account is suggested. The fracture model is used and validated with impact tests of iron ore pellets. In the validation experiment, iron ore pellets are fired against a steel plate, and the percentage of fractured pellets at different impact velocities are measured. Finite element simulations of the experiment are carried out and the probability of pellets fracturing during impact are calculated and compared with the experimental results. The agreement between the experiments and numerical simulations shows the validity of the model.
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| 19. |
- Huo, J., et al.
(författare)
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Realization of shorter substitutes for a percussive rock drill hammer by periodic redistribution of its mass and compliance
- 2023
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Ingår i: International Journal of Impact Engineering. - : Elsevier. - 0734-743X .- 1879-3509. ; 180
-
Tidskriftsartikel (refereegranskat)abstract
- In percussive drilling of rock, use is made of high-amplitude short-duration force pulses generated through impacts of a hammer on a drill bit or a drill rod assembly. The mass of the hammer is crucial for the impact velocity and the impact frequency, while its length and variation of characteristic impedance control the duration and shape of the force pulses. High efficiency of conversion of kinetic impact energy to work results if the force pulses have suitable shapes and durations. Commonly, hammers of hydraulic rock drills are nearly uniform and generate nearly rectangular force pulses admitting high efficiencies. When space is constrained, short hammers are advantageous as they allow short rock drills. Here, therefore, shorter substitutes for uniform original hammers are sought. They should have the same mass as the original hammer, be shorter than the original hammer, and generate approximately the same impact force history as the original hammer. A method based on 1D theory is developed for realization of substitute hammers with axially periodic characteristic impedance. Impact force histories generated by such hammers agree well with that generated by a uniform original hammer in a 1D context, and also in a 3D context if 3D effects are small enough. Also, for bit-rock interaction conditions from soft to hard, the efficiencies obtained by use of substitute hammers agree well with that obtained by use of the original hammer.
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| 20. |
- Karthikeyan, K., et al.
(författare)
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Optimal fibre architecture of soft-matrix ballistic laminates
- 2016
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Ingår i: International Journal of Impact Engineering. - : Elsevier. - 0734-743X .- 1879-3509. ; 88, s. 227-237
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Tidskriftsartikel (refereegranskat)abstract
- Soft-matrix ballistic laminates (such as those composed of fibres of Ultra High Molecular-Weight Polyethylene, e.g. Dyneema® HB26 and Spectra Shield) find extensive use as catching type armour systems. The relationship between the lay-up of these laminates with respect to the observed failure mechanisms has not been empirically investigated in the open literature, and is the subject of this work. Lay-ups are characterised by two parameters: (i) sequencing (or interply lay-up angle) θ¯ and (ii) in-plane anisotropy β, and can be mapped on to θ¯-β space. Four geometries that lie at the extrema of this parameter space are designed, built and tested. Testing is through ball bearing impact on circular clamped plates. The anisotropy (β) is coupled to the macroscopic response of the plates, while sequencing (θ¯) is coupled to the microscopic response. Penetration velocity is strongly affected by pull-out at the boundary, and in the present study this is shown to account for two-thirds of the ballistic resistance. The results have implications for validation testing on scaled samples, predictive modelling and simulation, and armour design.
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| 21. |
- Kroon, Martin, Professor, 1974-, et al.
(författare)
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Impact testing of high-density polyethylene structure
- 2024
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Ingår i: International Journal of Impact Engineering. - : Elsevier. - 0734-743X .- 1879-3509. ; 192
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Tidskriftsartikel (refereegranskat)abstract
- High strain -rate testing of high -density polyethylene is the focus of the present work. This testing is accomplished by two types of experimental testing: uniaxial tensile testing using standard testing technique, and impact testing of a 3D structure with non -trivial geometry. Both the uniaxial tests and the impact tests were evaluated using a material model suited for rate -dependent inelasticity of polymers that has been developed. In the uniaxial tensile tests, a maximum strain -rate of about 28/s was attained. In the impact tests, strain -rates of the order of 100/s and beyond were predicted in the analyses. The impact tests were simulated and analysed by use of finite element simulations. Coupled Eulerian-Lagrangian (CEL) analyses were employed for some of the tests where there was an interaction between the compressed structure and air trapped inside it. Overall, the simulations were able to reproduce the outcome from the experiments well. In particular, the deformation scenarios in the impact tests for different loading situations could be reproduced.
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| 22. |
- Lidén, Ewa, et al.
(författare)
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Deformation and fracture of a long-rod projectile induced by an oblique moving plate : Numerical simulations
- 2012
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 40-41, s. 35-45
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Tidskriftsartikel (refereegranskat)abstract
- Simulations have been performed to evaluate the possibility of reproducing the fragmentation of a long-rod projectile impacted by a moving oblique plate. When the moving plate slides along the projectile, fractures due to shear loading may occur in the projectile. Therefore, a fracture model suggested by Xue–Wierzbicki was used for the projectile together with the Johnson–Cook strength model. This fracture model is based on an equivalent plastic strain of fracture which depends on a stress triaxiality and a deviatoric stress parameter. The results of the simulations were compared with experimental results of a preceding study in which the impact conditions were varied in such a way that the projectile fractured in some but not all tests. The comparisons show that the simulations reproduced the fractures in the projectile well. Also, the transition from a deformed non-fractured to a severely fractured projectile was captured. The benefit of including the deviatoric stress parameter and the mechanisms leading to fragmentation of the projectile are discussed.
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| 23. |
- Lidén, Ewa, et al.
(författare)
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Deformation and fracture of a long-rod projectile induced by an oblique moving plate : Experimental test
- 2011
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 38:12, s. 989-1000
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Tidskriftsartikel (refereegranskat)abstract
- The influence of projectile length to diameter ratio (15, 30 and 45), plate thickness (0.5, 1 and 2 projectile diameters), projectile velocity (1500, 2000 and 2500 m/s) and plate velocity (−300 to 300 m/s) on the interaction between long-rod tungsten projectiles and oblique steel plates (obliquity 60°) was studied experimentally in small-scale reverse impact tests. The residual projectiles and their motions were characterised in terms of changes in length, velocity, angular momentum, linear momentum and kinetic energy. The parameters found to have the largest influence on the disturbance of the projectile were the plate velocity, in particular its direction, and the thickness of the plate. In the ranges studied, the influence of length to diameter ratio and of projectile velocity were found to be less important.
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| 24. |
- Lidén, Ewa, et al.
(författare)
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Effect of thin oblique moving plates on long rod projectiles : a reverse impact study
- 2006
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 32:10, s. 1696-1720
-
Tidskriftsartikel (refereegranskat)abstract
- The geometry and motion of long rod projectiles after penetrating thin obliquely oriented and moving armour plates were studied. Plates moving in their normal directions towards as well as away from the projectile (scalar product of velocities negative and positive, respectively) were considered. The influences of plate velocity and obliquity (angle between the normal of the plate and the axis of the projectile) were investigated through small-scale reverse impact tests with tungsten projectiles of length 30 mm and diameter 2 mm, and with 2 mm-thick steel plates. The obliquity (30°, 60° and 70°) and the plate velocity (300 to −300 m/s) were varied systematically for a projectile velocity of 2000 m/s. The disturbing effect of the plate on the projectile was characterised in terms of changes in length, velocity, angular momentum, linear momentum and kinetic energy. Plates with obliquity 60–70° moving away from the projectiles with velocity 200–300 m/s were found to cause extensive fragmentation of the projectile and to have large disturbing effects in terms of all measures used.
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| 25. |
- Lundberg, Bengt, et al.
(författare)
-
Biconvex versus bilinear force-penetration relationship in percussive drilling of rock
- 2017
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 100, s. 7-12
-
Tidskriftsartikel (refereegranskat)abstract
- Because of variability of the force vs. penetration relationship (FPR) from one blow to another in percussive drilling, and difficulty to predict FPRs under such conditions, use is commonly made of simple FPR models, such as the bilinear one defined by its loading/unloading slopes. Here a biconvex model with an added parameter representing convexity is considered. One aim is to study the effect of convexity on maximal penetration, maximal force and efficiency. Another is to assess, with the biconvex FPR as an example, how well a bilinear FPR can be used to approximate one that is nonlinear. A simple percussive top-hammer drill model is considered, comprising a hammer, a drill rod and a bit with the same characteristic impedance. The maximal penetration is found to decrease and the maximal force to increase with increasing convexity. The efficiency has a maximum for a finite hammer length (incident wave duration), and the highest maximal efficiency is obtained for a linear FPR With increasing convexity, the maximal efficiency decreases and occurs for shorter hammers (incident waves). The bilinear approximation of a biconvex FPR accurately predicts the position of the maximum in efficiency, even for large convexity, but somewhat overestimates its height and width.
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| 26. |
- Lundberg, Bengt, et al.
(författare)
-
Effect of pre-impact waves in an elastic rod on coefficient of restitution
- 2021
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 151
-
Tidskriftsartikel (refereegranskat)abstract
- The effect of pre-impact waves on the coefficient of restitution (COR) in longitudinal impact of a uniform elastic rod on a linear elastic or bilinear inelastic spring supported by a fix rigid wall is studied. The spring models may represent the quasi-static response of, e.g., a small elastic cylinder or the inelastic bit-rock interaction in percussive drilling. The rod is accelerated to its impact momentum either by a force of relatively short duration which generates significant pre-impact waves in the rod (pre-impact wave case) or by a force of long duration which does not generate such waves (reference case). Analytical and numerical results based on the 1D wave equation are presented in dimensionless form. It is shown that the work performed by the accelerating force is much larger in the pre-impact wave case than in the reference case, and that the COR generally depends on spring stiffness, elasticity parameter, initial rod-spring separation, and wave transit time. In the pre-impact wave case, the COR may be significantly larger or smaller than in the reference case. It may also be significantly larger or smaller than unity. A pre-impact wave mechanism leading to super rebounds with COR up to 1.4261 is described.
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| 27. |
- Lundberg, Bengt, et al.
(författare)
-
Optimal wave shape with respect to efficiency in percussive drilling with detachable drill bit
- 2015
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 86, s. 179-187
-
Tidskriftsartikel (refereegranskat)abstract
- Abstract The problem of finding the optimal incident wave of given duration that maximizes the efficiency of conversion of wave energy into work in percussive drilling with detachable drill bit is considered. The drill rod is modelled as 1D linearly elastic and the drill bit as a rigid mass. The bit/rock interaction is described by a history-dependent force versus penetration relation with different constant slopes for primary loading and unloading/reloading. A functional expressing the dependence of the efficiency on the shape of an arbitrary incident wave of given duration is derived and maximized. For short incident waves, there is a weak influence of the bit mass on the optimal wave shape which is nearly rectangular. For longer incident waves, there is a strong influence of the bit mass on the optimal wave shape which significantly differs from rectangular. The efficiencies for optimal waves approach those for rectangular waves for short waves. For long waves they approach or assume values which are independent of wave duration but decrease with increasing bit mass. Relative to commonly-used rectangular waves significant increase in efficiency can be achieved through optimization of the wave shape if the wave is not too short. Optimal incident waves can be realized accurately, e.g., by piezoelectric means.
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| 28. |
- Lundberg, Bengt, et al.
(författare)
-
Optimal wave with respect to efficiency in percussive drilling with integral drill steel
- 2010
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 37:8, s. 901-906
-
Tidskriftsartikel (refereegranskat)abstract
- The problem considered is that of finding the optimum wave of given finite duration that maximizes the efficiency of conversion of wave energy into work in percussive drilling with integral drill steel (drill rod with integrated bit). A 10 model is used for the drill rod, and the bit-rock interaction is represented by a piecewise linear force versus penetration relation with different penetration resistances for primary loading and for unloading/reloading. A functional expressing the dependence of the efficiency on the incident wave is derived and maximized. The optimal incident wave has exponential shape with time constant for the growth rate equal to the characteristic response time of the percussive drill system, including the rock. The maximal efficiency increases monotonously with the duration of the optimal wave. It approaches zero for very short waves and unity for very long waves. Optimal waves of short duration are close to rectangular while those of long duration approach the semi-infinite exponential wave derived by Long in the 1960s. Optimal waves of medium or longer duration give significantly higher efficiencies than commonly used rectangular waves of the same duration.
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| 29. |
- Lundberg, Patrik, et al.
(författare)
-
Impact of conical tungsten projectiles on flat silicon carbide targets : Transition from interface defeat to penetration
- 2006
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 32:11, s. 1842-1856
-
Tidskriftsartikel (refereegranskat)abstract
- Normal impact of conical tungsten projectiles on flat silicon carbide targets was studied experimentally and numerically for half apex angles 5° and 5–15°, respectively, and comparisons were made with cylindrical projectiles. A 30 mm powder gun and two 150 kV and four 450 kV X-ray flashes were used in the impact tests. The numerical simulations were run with the Autodyn code in two steps. In the first, the surface loads were determined for different impact velocities under assumed condition of interface defeat. In the second, these surface loads were applied to the targets in order to obtain critical states of damage and failure related to the transition between interface defeat and penetration, and the corresponding critical velocities. In the impact tests, interface defeat occurred below a transition velocity, which was significantly lower for the conical than for the cylindrical projectiles. Above the transition velocity, the initial penetration of conical projectiles differed markedly from that usually observed for cylindrical projectiles. It occurred along a cone-shaped surface crack, qualitatively corresponding to surface failure observed in the simulations. The transition velocity for the conical projectile was found to be close to the critical velocity associated with this surface failure.
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| 30. |
- Lundberg, Patrik, et al.
(författare)
-
Transition between interface defeat and penetration for tungsten projectiles and four silicon carbide materials
- 2005
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 31:7, s. 781-792
-
Tidskriftsartikel (refereegranskat)abstract
- Armour systems containing high-quality ceramics may be capable of defeating armour-piercing projectiles on the surfaces of these hard materials. This capability, named interface defeat, has been studied for four different silicon carbide ceramic materials, viz., SiC–B, SiC–N, SiC–SC–1RN and SiC–HPN by use of a light-gas gun and a small-scale reverse impact technique. The velocities of a tungsten projectile marking the transition between interface defeat and penetration have been determined and compared with the Vickers hardness and fracture toughness of the ceramic materials. It is found that the transition velocity increases with the fracture toughness but not with the Vickers hardness. This indicates that, under the prevailing conditions, fracture may have had more influence than plastic flow on the transition. As a consequence, the observed transition velocities may not be the maximum ones achievable, at least not for SiC–B, SiC–N and SiC–SC–1RN. By suppression of the initiation and propagation of cracks through increase of the confining pressure, it may be possible to increase the transition velocities.
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| 31. |
- Lönn, David, 1979-, et al.
(författare)
-
An approach to robust optimization of impact problems using random samples and meta-modelling
- 2010
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 37:6, s. 723-734
-
Tidskriftsartikel (refereegranskat)abstract
- Conventionally optimized structures may show a tendency to be sensitive to variations, for instance in geometry and loading conditions. To avoid this, research has been carried out in the field of robust optimization where variations are taken into account in the optimization process. The overall objective is to create solutions that are optimal both in the sense of mean performance and minimum variability. This work presents an alternative approach to robust optimization, where the robustness of each design is assessed through multiple sampling of the stochastic variables at each design point. Meta-models for the robust optimization are created for both the mean value and the standard deviation of the response. Furthermore, the method is demonstrated on an analytical example and an example of an aluminium extrusion with quadratic cross-section subjected to axial crushing. It works well for the chosen examples and it is concluded that the method is especially well suited for problems with a large number of random variables, since the computational cost is essentially independent of the number of random variables. In addition, the presented approach makes it possible to take into consideration variations that cannot be described with a variable. This is demonstrated in this work by random geometrical perturbations described with the use of Gaussian random fields.
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| 32. |
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| 33. |
- Mohr, Dirk, et al.
(författare)
-
Evaluation of stress-strain curve estimates in dynamic experiments
- 2010
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 37:2, s. 161-169
-
Tidskriftsartikel (refereegranskat)abstract
- Accurate measurements of the forces and velocities at the boundaries of a dynamically loaded specimen may be obtained using split Hopkinson pressure bars (SHPB) or other experimental devices. However, the determination of a representative stress-strain curve based on these measurements can be challenging. Due to transient effects, the stress and strain fields are not uniform within the specimen. Several formulas have been proposed in the past to estimate the stress-strain curve from dynamic experiments. Here, we make use of the theoretical solution for the waves in an elastic specimen to evaluate the accuracy of these estimates. it is found that it is important to avoid an artificial time shift in the processing of the experimental data. Moreover, it is concluded that the combination of the output force based stress estimate and the average strain provides the best of the commonly used stress-strain curve estimates in standard SHPB experiments.
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| 34. |
- Mousavi, Saed, et al.
(författare)
-
Non-equilibrium split Hopkinson pressure bar procedure for non-parametric identification of complex modulus
- 2005
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 31:9, s. 1133-1151
-
Tidskriftsartikel (refereegranskat)abstract
- A split Hopkinson pressure bar procedure was developed for non-parametric identification of complex modulus under conditions of non-equilibrium and axially non-uniform stress. Two simplified procedures were also established. The first requires low frequency and/or short specimen. The second, identical to a classical procedure based on equilibrium, requires that also the specimen-to-bar characteristic impedance ratio be low. Both overestimate the magnitude of the complex modulus, the second even at low frequencies. Tests were carried out with polymethyl methacrylate and aluminium bars and with polypropylene specimens having diameter 20 mm and lengths 10, 20, 50 and 100 mm. The complex moduli identified are in good to fair agreement with published results up to 10 kHz for all specimens with polymethyl methacrylate bars and for the 10–50 mm specimens with aluminium bars. The quality of the results is sensitive to truncation and to imperfect contact at the bar-specimen interfaces.
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| 35. |
- Nilsson, Kristina
(författare)
-
Effects of elastic unloading on multiple necking in tensile bars
- 2004
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 1879-3509 .- 0734-743X. ; 30:10, s. 1353-1367
-
Tidskriftsartikel (refereegranskat)abstract
- The deformation of round tensile bars subject to dynamic loading is studied numerically using two different plasticity theories: J(2)-flow theory and J(2)-deformation theory. Time development of neck localization is visualized and a comparison of the necking patterns obtained using the two different constitutive models is presented. The possibility of using J(2)-deformation theory as an alternative to J(2)-flow theory to model the dynamic behaviour of tensile bars subject to high rate deformation is investigated and the effects of elastic unloading are discussed. Influence of specimen geometry and initial thickness imperfections is also considered. The effects of elastic unloading on multiple necking are of particular interest, since elastic unloading may serve as a restricting factor regarding the growth of multiple neck localizations. In the present analysis, multiple necking is found to appear using both J(2)-flow theory and J(2)-deformation theory. The use of stubby bars leads to similar necking patterns when comparing the results obtained with J(2)-flow theory and J(2)-deformation theory. For slender bars subject to certain loading conditions a larger number of necks is obtained when J(2)-deformation theory is applied rather than J(2)-flow theory. In addition, results of simulations near the ideal-plastic limit are discussed as well as results where the yield stress is varied. (C) 2003 Elsevier Ltd. All rights reserved.
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| 36. |
- Odd-Geir, Lademo, et al.
(författare)
-
A model for process-based crash simulation
- 2008
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 35:5, s. 376-388
-
Tidskriftsartikel (refereegranskat)abstract
- Manufacturing of a bumper system from aluminium extrusions often involves series of forming operations performed in the soft W-temper condition, and then artificially age-hardening of the components to the material's peak hardness T6 condition. It is probable that proper finite element (FE) modelling of the crash performance of the resulting systems must rely upon a geometry obtained from an FE model following the process route, i.e., including simulation of all major forming operations. The forming operations also result in an inhomogeneous evolution of some internal variables (among others the effective plastic strain) within the shaped components. Results from tensile tests reveal that plastic straining in W-temper leads to a significant change of the T6 work-hardening curves. In addition, the tests show that the plastic pre-deformation causes a reduction of the elongation of the T6 specimens. In the present work, these process effects have been included in a user-defined elastoplastic constitutive model in LS-DYNA incorporating a state-of-the-art anisotropic yield criterion, the associated flow rule and a non-linear isotropic work hardening rule as well as some ductile fracture criteria. A first demonstration and assessment of the modelling methodology is shown by ‘through-process analysis' of two uniaxial tensile test series. The industrial use and relevance of the modelling technique is subsequently demonstrated by a case study on an industrial bumper beam system.
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| 37. |
- Renström, René, et al.
(författare)
-
Self-similar flow of a conical projectile on a flat target surface under conditions of dwell
- 2009
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 36:2, s. 352-362
-
Tidskriftsartikel (refereegranskat)abstract
- In order to investigate the state of stress in a target material under conditions of interface defeat or dwell it is necessary to determine the load intensity at the interface of the flowing projectile material and the target. Previous studies for a cylindrical projectile geometry at normal impact under stationary conditions show that the load can be considered to be composed of three components, viz., those of inertia, compressibility and yield strength of the projectile material. In order to determine the influence of projectile shape, a conical projectile in axi-symmetric impact on a ridged, friction-free surface is studied by use of an analytical model for self-similar flow and numerical Autodyn simulations. It is shown how the maximum load intensity, and the position of the maximum, depends on the apex angle. Both the self-similar model and the Autodyn simulations show that the contribution to the load intensity from compressibility is positive below and negative above apex angles 80°. The influence of yield strength on the load intensity depends only weakly on the apex angle and therefore corresponds to that for a cylindrical projectile.
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| 38. |
- Renström, René, et al.
(författare)
-
Stationary contact between a cylindrical metallic projectile and a flat target surface under conditions of dwell
- 2004
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 30:6, s. 1265-1282
-
Tidskriftsartikel (refereegranskat)abstract
- Armour systems capable of defeating an incoming projectile on the surface of a ceramic target have been reported. This capability, called interface defeat or dwell, signifies that the projectile material is forced to flow radially on the surface of the target without penetrating significantly. Under such flow conditions, the hydrodynamic pressure is normally the most important part of the normal load on the target surface. Therefore, projectile properties such as yield strength and compressibility are commonly ignored or assumed to contribute only marginally. In order to investigate the effects of these properties, an analytical expression was derived for the normal load from a cylindrical metallic projectile impacting on a flat, rigid and friction-free surface, which includes the contributions from yield strength and compressibility in addition to that of inertia. At an impact velocity representative of today's ordinance velocities, the contributions to load intensity on the axis from yield strength and compressibility were found to be 15% and 3.4%, respectively, of that of inertia. The analytical results and Autodyn-2D numerical simulations show good agreement within a projectile radius from the axis.
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| 39. |
- Tilert, Dan, et al.
(författare)
-
Measurement of Explosively Induced Movement and Spalling of Granite Model Blocks
- 2007
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 34:12, s. 1936-1952
-
Tidskriftsartikel (refereegranskat)abstract
- Charges of high explosives have been buried (countersunk) in granite blocks and detonated. This article describes the measurement and evaluation of the shock wave propagating through the granite blocks. It also demonstrates how the shock wave data can be used to improve computer simulations of granite's behaviour. The overall goal has been to investigate how granite withstands penetrating weapons, that first penetrate the ground and then detonates within the created cavity. Several variables have been investigated. It is shown that water content of the granite can increase the shock wave amplitude with up to a factor 2, and a crack in the granite often attenuates the shock wave amplitude with a factor 4 or more. Also, the granite block thickness needed to prevent internal crack formation has been investigated.
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| 40. |
- Unosson, Mattias, et al.
(författare)
-
Projectile penetration and perforation of high performance concrete: Experimental results and macroscopic modelling
- 2006
-
Ingår i: International Journal of Impact Engineering. - : Pergamon Press. - 0734-743X .- 1879-3509. ; 32:7, s. 1068-1085
-
Tidskriftsartikel (refereegranskat)abstract
- Experiments and simulations of penetration and perforation of high performance concrete targets by steel projectiles have been carried out. A Doppler radar monitored the projectile pre-impact velocity history and a high-speed camera captured the projectile residual velocity. A continuum mechanical approach and the finite-element method were used for the simulations. The targets were modelled with the K&C concrete model. For perforation the computational results show acceptable agreement with the experimental results, but not for penetration. The article is concluded with suggestions on how to better model concrete material for the current application.
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| 41. |
- Welch, Ken, et al.
(författare)
-
Low frequency limitations of the split Hopkinson pressure bar method for identification of complex modulus
- 2007
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 34:6, s. 1036-1046
-
Tidskriftsartikel (refereegranskat)abstract
- Low frequency limitations of a recently developed method for identification of complex modulus utilizing the split Hopkinson pressure bar (SHPB) technique were investigated using computer simulations. Specifically, the effects of truncation, noise and discretization were examined. It was shown that the low frequency limitation of the method generally corresponds to the inverse of the length of the time signal. Further, it was shown that all three factors have an effect on the low frequency accuracy of the method and that careful consideration of these factors is necessary to optimize the capability of the method. Finally, it was shown how averaging techniques can be implemented to reduce the undesirable effects of truncation and noise.
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| 42. |
- Zakrisson, Björn, et al.
(författare)
-
Numerical simulations of blast loads and structural deformation from near-field explosions in air
- 2011
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X .- 1879-3509. ; 38:7, s. 597-612
-
Tidskriftsartikel (refereegranskat)abstract
- Numerical simulations of air blast loading in the near-field acting on deformable steel plates have been performed and compared to experiments. Two types of air blast setups have been used, cylindrical explosive placed either in free air or in a steel pot. A numerical finite element convergence study of the discretisation sensitivity for the gas dynamics has been performed, with use of mapping results from 2D to 3D in an Eulerian reference frame. The result from the convergence study served as a foundation for development of the simulation models. Considering both air blast setups, the numerical results under predicted the measured plate deformations with 9.4–11.1%. Regarding the impulse transfer, the corresponding under prediction was only 1.0–1.6%. An influence of the friction can be shown, both in experiments and the simulations, although other uncertainties are involved as well. A simplified blast model based on empirical blast loading data representing spherical and hemispherical explosive shapes has been tested as an alternative to the Eulerian model. The result for the simplified blast model deviates largely compared to the experiments and the Eulerian model. The CPU time for the simplified blast model is however considerably shorter, and may still be useful in time consuming concept studies. All together, reasonable numerical results using reasonable model sizes can be achieved from near-field explosions in air.
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| 43. |
- Berzi, P, et al.
(författare)
-
Identification of a percussive drill rod joint from its response to stress wave loading
- 1996
-
Ingår i: INTERNATIONAL JOURNAL OF IMPACT ENGINEERING. - 0734-743X. ; 18:3, s. 281-290
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Tidskriftsartikel (refereegranskat)abstract
- In percussive drilling of rock, elastic stress waves are generated in a drill string through repeated axial impacts by the hammer of a rock drill. For holes deeper than a few meters, several drill rods are commonly joined by means of cylindrical coupling
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| 44. |
- Hogström, Per, 1979, et al.
(författare)
-
An experimental and numerical study of the effects of length scale and strain state on the necking and fracture behaviours in sheet metals
- 2009
-
Ingår i: International Journal of Impact Engineering. - 0734-743X. ; 36:10-11, s. 1194-1203
-
Tidskriftsartikel (refereegranskat)abstract
- Within sheet metal forming, crashworthiness analysis in the automotive industry and ship research on collision and grounding, modelling of the material failure/fracture, including the behaviour at large plastic deformations, is critical for accurate failure predictions. In order to validate existing failure models used in finite element (FE) simulations in terms of dependence on length scale and strain state, tests recorded with the optical strain measuring system ARAMIS have been conducted. With this system, the stress–strain behaviour of uniaxial tensile tests was examined locally, and from this information true stress–strain relations were calculated on different length scales across the necking region. Forming limit tests were conducted to study the multiaxial failure behaviour of the material in terms of necking and fracture. The failure criteria that were verified against the tests were chosen among those available in the FE software Abaqus and the Bressan–Williams–Hill (BWH) criterion proposed by Alsos et al, 2008. The experimental and numerical results from the tensile tests confirmed that Barba’s relation is valid for handling stress–strain dependence on the length scale used for strain evaluation after necking. Also, the evolution of damage in the FE simulations was related to the processes ultimately leading to initiation and propagation of a macroscopic crack in the final phase of the tensile tests. Furthermore, numerical simulations using the BWH criterion for prediction of instability at the necking point showed good agreement with the forming limit test results. The effect of pre-straining in the forming limit tests andthe FE simulations of them is discussed.
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| 45. |
- Leppänen, Joosef, 1970
(författare)
-
Concrete subjected to projectile and fragment impacts: Modelling of crack softening and strain rate dependency in tension
- 2006
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X. ; 32:11, s. 1828-1841
-
Tidskriftsartikel (refereegranskat)abstract
- This paper deals with modelling of plain concrete in tension. The aim is to improve the accuracy of the numerical analyses for projectile and fragment impacts on concrete. A bi-linear crack softening law and a strain rate-dependent law are implemented in the hydrocode AUTODYN. Parametric studies are made, and numerical analyses are compared with experiments conducted and with experiments found in the literature. The depth of penetration is mainly dependent on the compressive strength of the concrete. However, to correctly model spalling, cracking and scabbing in concrete, the tensile strength, fracture energy, and strain rate in tension are very important. It is shown that the accuracy of the results in the numerical analyses of concrete subjected to projectile and fragment impacts was improved, when using a bi-linear softening law and the modified strain rate dependency for tension.
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| 46. |
- Leppänen, Joosef, 1970
(författare)
-
Experiments and numerical analyses of blast and fragment impacts on concrete.
- 2005
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X. ; 31:7, s. 843-860
-
Tidskriftsartikel (refereegranskat)abstract
- Concrete structures are commonly used as protective structures. An important issue is how the blast wave and fragment impacts from an explosion affect the concrete. It is well known that the fragments penetrate or even perforate the structure. Moreover, spalling occurs in the impact zone and scabbing may occur on the reverse side of a wall that receives an impact. However, knowledge of how the blast wave and fragment impacts influence the material properties of concrete is quite limited. Experiments and numerical analyses were carried out to examine the extent to which the concrete, at various distances, is affected by the blast wave and fragment impacts. The fragments, which were spherical, were shot against thick concrete blocks by using the explosives octol or hexotol; the fragment velocity was approximately 1 650 m/s. After the concrete blocks were shot, the depths of penetration and spalling were measured. Next, the concrete blocks were cut into halves, and the global macro cracking could be observed. To study how the material properties of concrete were influenced, uniaxial compressive and splitting tensile tests were carried out on cylinders drilled from selected positions in the block. Furthermore, specimens from the blocks were thin-ground to facilitate analysing the micro cracking with a microscope. The experiments and numerical analyses presented here showed that the damage in the concrete, from the blast wave and fragment impacts, is localized in the impact zone. The concrete below this zone, at a depth of approximately twice the depth of the maximum penetration, was hardly affected at all by the blast wave and fragment impacts. This indicates that it is possible to distinguish between the global load effects and the local damage effects that are caused by the fragment impacts. Consequently, it may be possible to separate the loads, at the design stage, from a blast wave and fragment impacts.
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| 47. |
- Li, Xiaogai, et al.
(författare)
-
Behind helmet blunt trauma induced by ballistic impact : a computational model
- 2016
-
Ingår i: International Journal of Impact Engineering. - : Elsevier. - 0734-743X. ; 91, s. 56-67
-
Tidskriftsartikel (refereegranskat)abstract
- Behind helmet blunt trauma (BHBT) has emerged as a serious injury type experienced by soldiers in battlefields. BHBT has been found to range from skin lacerations to brain damage and extensive skull fracture. It has been believed that such injuries are caused by forces transmitted from the helmet's back face deformation (BFD), which result in local deformations of the skull and translation or rotation of the head, leading to brain injuries. In this study, head injury risks resulting from the BFD of the Advanced Combat Helmet (ACH) under ballistic impact are evaluated using finite element simulations. The head model developed at KTH in Sweden is adopted, and a helmet shell model (including foam pads) is constructed. The examined mechanical parameters include the maximum von Mises stress in the skull, pressure (mean normal stress) and maximum principal strain in the brain tissue, contact force, and head acceleration. The influences of the foam pad hardness, stand-off distance, helmet shell thickness, and impact direction on head injury risks are studied. It is found that a softer foam pad offers a better protection, but the foam pad cannot be too soft. Also, it is shown that a slightly larger stand-off distance leads to a significant reduction in head injury. In addition, the simulation results reveal that an increase in the helmet thickness reduces the injury risk. It is further observed that a 45-degree oblique frontal impact results in a lower head injury risk than a 90-degree frontal impact. Moreover, for a helmet protected head under ballistic impact, it is seen that a high risk of skull fracture does not necessarily mean an equally high risk of injury to the brain tissue. The predictions from the current model of a helmeted head under ballistic impact agree with experimental findings independently obtained by others. The newly developed model provides a useful tool for studying injury mechanisms of BHBT and evaluating the existing standards for testing and designing combat helmets.
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| 48. |
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| 49. |
- Nyström, Ulrika, 1981, et al.
(författare)
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Comparative numerical studies of projectile impacts on plain and steel-fibre reinforced concrete
- 2011
-
Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X. ; 38:2-3, s. 95-105
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Tidskriftsartikel (refereegranskat)abstract
- The enhanced energy absorption characteristics of fibre-reinforced concrete, compared to plain concrete, has in experimentally studies been shown to improve the projectile resistance and motivate its increased usage in protective structures. However, the high cost of undertaking experiments and the high parameter variation and dependency of the experimental setups and results, respectively, make it difficult to draw generic conclusions of how the addition and increased amount of fibres affects the local damage caused by projectile impact, which motivates the use of numerical simulations to study this. The numerical hydrocode AUTODYN was used in a qualitative study of how the addition of different amounts of fibres, modelled as different post-crack relations, influence the depth of penetration and crater formation on the front and rear face of a concrete target. Fibres added to the concrete mix had a minor influence on the depth of penetration while the crater size on both front and rear faces of the target decreases. The crack propagation beyond the crater on the front face was also reduced when fibres were added to the concrete. An increased amount of fibres in the concrete showed no effect on the size of the front-face crater, but led to further decreased size of the crater on the rear face of the concrete cylinder. It is concluded that the scabbing crater can be reduced in size and prevented by usage of fibre-reinforced concrete even if the depth of penetration is only slightly less than to penetration depth in plain concrete.
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- Nyström, Ulrika, 1981, et al.
(författare)
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Numerical studies of the combined effects of blast and fragment loading
- 2009
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Ingår i: International Journal of Impact Engineering. - : Elsevier BV. - 0734-743X. ; 36:8, s. 995-1005
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Tidskriftsartikel (refereegranskat)abstract
- The well-known synergetic effect of blast and fragment loading, observed in numerous experiments, is often pointed out in design manuals for protective structures. However, since this synergetic effect is not well understood it is often not taken into account, or is treated in a very simplified manner in the design process itself. A numerical-simulation tool has been used to further study the combined blast and fragment loading effects on a reinforced concrete wall. Simulations of the response of a wall strip subjected to blast loading, fragment loading, and combined blast and fragment loading were conducted and the results were compared. Most damage caused by the impact of fragments occurred within the first 0.2 ms after fragments' arrival, and in the case of fragment loading (both alone and combined with blast) the number of flexural cracks formed was larger than in the case of blast loading alone. The overall damage of the wall strip subjected to combined loading was more severe than if adding the damages caused by blast and fragment loading treated separately, which also indicates the synergetic effect of the combined loading. (C) 2009 Elsevier Ltd. All rights reserved.
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